Band saw with improved safety system

ABSTRACT

A band saw including a frame, at least two, spaced apart, rotatable wheels supported by the frame, a blade looped around the wheels, where rotation of at least one wheel causes the blade to move around the wheels, a detection system adapted to detect a dangerous condition between a person and the blade, and a reaction system configured to engage and stop the blade within 10 milliseconds after detection of the dangerous condition is disclosed. The reaction system may be configured to cut and grip the blade.

COPYRIGHT NOTICE

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

The computer program listing identified herein is subject to copyrightprotection.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/146,527, filed May 15, 2002, which in turn claims the benefit of andpriority from U.S. Provisional Patent Application Ser. No. 60/292,100,filed May 17, 2001, both of which are herein incorporated by reference.

This application hereby incorporates by reference the following U.S.patent application Ser. No. 09/676,190, filed Sep. 29, 2000.

This application also hereby incorporates by reference the following PCTpatent application: PCT/US00/26812, filed Sep. 29, 2000.

This application further incorporates by reference the following U.S.provisional patent applications: Ser. No. 60/157,340, filed Oct. 1,1999, Ser. No. 60/182,866, filed Feb. 16, 2000, Ser. No. 60/225,056,filed Aug. 14, 2000, Ser. No. 60/225,057, filed Aug. 14, 2000, Ser. No.60/225,058, filed Aug. 14, 2000, Ser. No. 60/225,059, filed Aug. 14,2000, Ser. No. 60/225,089, filed Aug. 14, 2000, Ser. No. 60/225,094,filed Aug. 14, 2000, Ser. No. 60/225,169, filed Aug. 14, 2000, Ser. No.60/225,170, filed Aug. 14, 2000, Ser. No. 60/225,200, filed Aug. 14,2000, Ser. No. 60/225,201, filed Aug. 14, 2000, Ser. No. 60/225,206,filed Aug. 14, 2000, Ser. No. 60/225,210, filed Aug. 14, 2000, Ser. No.60/225,211, filed Aug. 14, 2000, Ser. No. 60/225,212, filed Aug. 14,2000, Ser. No. 60/233,459, filed Sep. 18, 2000, Ser. No. 60/270,011,filed Feb. 20, 2001, Ser. No. 60/270,941, filed Feb. 22, 2001, Ser. No.60/270,942, filed Feb. 22, 2001, Ser. No. 60/273,178, filed Mar. 2,2001, Ser. No. 60/273,177, filed Mar. 2, 2001, Ser. No. 60/273,902,filed Mar. 6, 2001, Ser. No. 60/275,594, filed Mar. 13, 2001, Ser. No.60/275,595, filed Mar. 13, 2001, Ser. No. 60/275,583, filed Mar. 13,2001, Ser. No. 60/279,313, filed Mar. 27, 2001, and Ser. No. 60/292,081,filed May 17, 2001.

COMPUTER PROGRAM LISTING APPENDIX

A computer program listing is being submitted herewith via EFS-Web as aComputer Program Listing Appendix. The computer program listing is oneASCII text file entitled “sawbrk.txt”. The date of creation of the fileis Jun. 29, 2000, and the size of the file is 50602 bytes. That computerprogram listing (i.e., the “sawbrk.txt” ASCII text file) is herebyincorporated by reference.

FIELD

The present invention relates to band saws, and more particularly to aband saw with a high-speed safety system.

BACKGROUND

Band saws are a type of woodworking machinery used to cut workpieces ofwood, plastic, and other materials. Band saws include two, spaced-apartwheels, and a blade tightly looped around the wheels. The blade is madefrom a band of metal with teeth on one edge of the band. The blade movesaround the wheels when the wheels spin. Band saws also include a tableor work surface adjacent the blade and upon which workpieces are placed.A person uses the band saw by placing a workpiece on the table and thensliding the workpiece into the moving blade. Band saws present a risk ofinjury to users because the blade is exposed when in use. Furthermore,users often must place their hands very close to the blade to positionand move workpieces, which increases the chance that an injury willoccur.

The present invention provides a band saw with an improved safety systemthat is adapted to detect the occurrence of one or more dangerous, ortriggering, conditions during use of the band saw, such as when a user'sbody contacts the moving blade. When such a condition occurs, the safetysystem is actuated to limit or even prevent injury to the user.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of a band saw with a fast-actingsafety system according to the present invention.

FIG. 2 is a schematic side elevation of an embodiment of a band sawaccording to the present invention.

FIG. 3 shows a wheel mount with a capacitive coupling used in the bandsaw of FIG. 2.

FIG. 4 shows a wheel mount used in the band saw of FIG. 2.

FIG. 5 shows schematically a reaction subsystem that stops a blade bycutting and gripping the blade.

FIG. 6 is a schematic representation of one embodiment of a cutting pawland backing plate used in a reaction system that stops a blade bycutting the blade.

FIG. 7 is a schematic top view of another embodiment of cutting pawlsused in a reaction system to stop a blade by cutting the blade.

FIG. 8 is a schematic front view of the cutting pawls shown in FIG. 7.

FIG. 9 is a schematic circuit diagram of an electronic subsystem for thesafety system of FIG. 1, including an excitation system, a contact sensesystem and a firing system.

FIG. 10 is a schematic circuit diagram of a first alternative electronicsubsystem for the safety system of FIG. 1, including an excitationsystem, a contact sense system and a firing system.

FIG. 11 is a block diagram illustrating the arrangement of a secondalternative electronic subsystem.

FIG. 12 is a schematic diagram of an excitation system of the subsystemof FIG. 11.

FIG. 13 shows an exemplary attenuation in signal that occurs when thefinger of a user contacts a blade.

FIG. 14 is a schematic of a contact sense portion of the subsystem ofFIG. 11.

FIG. 15 is a schematic of a power supply of the subsystem of FIG. 11.

FIG. 16 is a schematic of a boost regulator portion and a firing portionof the subsystem of FIG. 11.

FIG. 17 is a schematic of a motor control portion of the subsystem ofFIG. 11.

FIG. 18 is a schematic of a rotation sensor portion of the subsystem ofFIG. 11.

FIG. 19 is a schematic of a user interface portion of the subsystem ofFIG. 11.

FIG. 20 is a block diagram of second and third alternative electronicsubsystems.

FIG. 21 is a schematic of an excitation system portion of the subsystemsof FIG. 20.

FIG. 22 is a schematic of a contact sense portion of the secondalternative subsystem of FIG. 20.

FIG. 23 is a schematic of a contact sense portion of the thirdalternative subsystem of FIG. 20.

FIG. 24 is a schematic of a power supply and firing system portion ofthe subsystems of FIG. 20.

DETAILED DESCRIPTION

A band saw according to the present invention is shown schematically inFIG. 1 and indicated generally at 10. Band saw 10 may be any of avariety of different types and configurations of band saws adapted forcutting workpieces, such as wood and plastic. Band saw 10 includes anoperative structure 12 having a blade 14 and a motor assembly 16 adaptedto drive the blade. Band saw 10 also includes a safety system 18configured to minimize the potential of a serious injury to a personusing the band saw. Safety system 18 is adapted to detect the occurrenceof one or more dangerous, or triggering, conditions during use of bandsaw 10. If such a dangerous condition is detected, safety system 18 isadapted to engage operative structure 12 to limit any injury to the usercaused by the dangerous condition.

Band saw 10 includes a suitable power source 20 to provide power tooperative structure 12 and safety system 18. Power source 20 may be anexternal power source such as line current, or an internal power sourcesuch as a battery. Alternatively, power source 20 may include acombination of both external and internal power sources. Furthermore,power source 20 may include two or more separate power sources, eachadapted to power different portions of band saw 10.

It will be appreciated that operative structure 12 may take any one ofmany different forms, depending on the type of band saw 10. As will bedescribed in more detail below, operative structure 12 typicallyincludes two, spaced-apart wheels and a table adjacent the wheels. Ablade 14, made from a band of metal with teeth along one edge of theband, is positioned around the wheels adjacent the table. Motor assembly16 includes one or more motors adapted to drive blade 14 by spinning atleast one of the wheels around which the blade is positioned.

Safety system 18 includes a detection subsystem 22, a reaction subsystem24 and a control subsystem 26. Control subsystem 26 may be adapted toreceive inputs from a variety of sources including detection subsystem22, reaction subsystem 24, operative structure 12 and motor assembly 16.The control subsystem may also include one or more sensors adapted tomonitor selected parameters of band saw 10. In addition, controlsubsystem 26 typically includes one or more instruments operable by auser to control the band saw. The control subsystem is configured tocontrol band saw 10 in response to the inputs it receives. Detectionsubsystem 22 is configured to detect one or more dangerous, ortriggering, conditions during use of band saw 10. For example, thedetection subsystem may be configured to detect that a portion of theuser's body is in contact with a portion of blade 14. In someembodiments, detection subsystem 22 may inform control subsystem 26 ofthe dangerous condition, which then activates reaction subsystem 24. Inother embodiments, the detection subsystem may be adapted to activatethe reaction subsystem directly. Various exemplary embodiments andimplementations of detection subsystem 22 are described in more detailin U.S. Provisional Patent Application Ser. No. 60/225,200, entitled“Contact Detection System for Power Equipment,” filed Aug. 14, 2000, bySD3, LLC, the disclosure of which is incorporated herein by reference.

Once activated in response to a dangerous condition, reaction subsystem24 is configured to quickly engage operative structure 12 to preventserious injury to the user. It will be appreciated that the particularaction to be taken by reaction subsystem 24 will vary depending on thetype of band saw 10 and/or the dangerous condition that is detected. Forexample, reaction subsystem 24 may be configured to do one or more ofthe following: stop the movement of blade 14 by cutting the blade and/orby gripping the blade, or by retracting the blade from its operatingposition.

The configuration of reaction subsystem 24 typically will vary dependingon which action(s) are taken. In the exemplary embodiment depicted inFIG. 1, reaction subsystem 24 is configured to stop the movement ofblade 14 and includes a brake mechanism 28, a biasing mechanism 30, arestraining mechanism 32, and a release mechanism 34. Brake mechanism 28is adapted to engage operative structure 12 under the urging of biasingmechanism 30. During normal operation of band saw 10, restrainingmechanism 32 holds the brake mechanism out of engagement with theoperative structure. However, upon receipt of an activation signal byreaction subsystem 24, the brake mechanism is released from therestraining mechanism by release mechanism 34, whereupon, the brakemechanism quickly engages at least a portion of the operative structureto bring the blade to a stop.

Control subsystem 26 includes one or more instruments that are operableby a user to control the motion of blade 14. Those instruments mayinclude start/stop switches, speed controls, etc. Control subsystem 26typically includes a logic controller connected to receive the user'sinputs via the instruments. The logic controller is also connected toreceive a contact detection signal from detection subsystem 22. Further,the logic controller may be configured to receive inputs from othersources, such as blade motion sensors, workpiece sensors, etc. In anyevent, the logic controller is configured to control operative structure12 in response to the user's inputs. However, upon receipt of a contactdetection signal from detection subsystem 22, the logic controlleroverrides the control inputs from the user and activates reactionsubsystem 24 to stop the motion of the blade. Various exemplaryembodiments and implementations of control subsystem 26, including thelogic controller, are described in more detail in U.S. ProvisionalPatent Application Ser. No. 60/225,059, entitled “Logic Control For FastActing Safety System,” filed Aug. 14, 2000 by SD3, LLC, and in U.S.Provisional Patent Application Ser. No. 60/225,094, entitled “MotionDetecting System For Use In Safety System For Power Equipment,” filedAug. 14, 2000 by SD3, LLC, the disclosures of which are hereinincorporated by reference.

A computer program listing for a controller as shown in FIG. 1 issubmitted herewith as a Computer Program Listing Appendix.

One embodiment of band saw 10 is shown specifically in FIG. 2. Itincludes a main housing 50 enclosing a pair of spaced-apart wheels 52and 54. Wheels 52 and 54 are supported for rotational movement byhousing 50. Housing 50 also typically encloses the wheels to prevent auser from touching them while they are spinning. The perimeter of eachwheel may be coated or covered in a high-friction material such asrubber, etc. A relatively thin, continuous loop, tooth-edged blade 14tightly encircles both wheels. Wheel 54 is driven by motor assembly 16(not shown in FIG. 2) so that it rotates in the direction of arrow 56.Rotating wheel 54 causes blade 14 to move, which in turn, causes wheel52 to rotate. Blade 14 moves adjacent table 58. A workpiece is cut bysliding the workpiece on table 58 into the teeth of moving blade 14between wheels 52 and 54. An upper blade-guide assembly 60 and a lowerblade-guide assembly 62 maintain the moving blade in a stable path.

Band saw 10 includes a detection subsystem 22 to detect when a person'sbody comes into contact with blade 14. Detection subsystem 22 is basedon the capacitance of a human body. It is believed that the capacitanceof a user's body, as measured through dry contact with a portion of theuser's body, is approximately 25-200 picofarads. That capacitance tendsto increase with increasing body size and with increased couplingbetween the user's body and an electrical ground. As a result of theinherent capacitance of a user's body, when the user touches blade 14,the capacitance of the user's body is electrically coupled to theinherent capacitance of the blade, thereby creating an effectivecapacitance that is larger than the inherent capacitance of the bladealone. Detection subsystem 22 is configured to measure or monitor thecapacitance of the blade, so that any substantial change in the measuredcapacitance would indicate contact between the user's body and theblade.

FIG. 3 shows a capacitive coupling that may be used as part of detectionsubsystem 22 in measuring the capacitance of the blade to detect contactbetween the blade and a person. In FIG. 3, wheel 54 is shown mounted onan arbor or shaft 72 by a bolt 74 and washer 76. Arbor 72 extendsthrough a hub 70 on the wheel, and bolt 74 is threaded into the arborand presses against hub 70 to hold the wheel on the arbor.

Arbor 72 is supported for rotational movement by bearings 80 and 82,which are mounted in a portion of housing 50, and which are spaced alongthe length of the arbor. Bearings 80 and 82 do not directly contactarbor 72 or wheel 54. Rather, arbor 72 and wheel 54 are electricallyisolated from bearings 80 and 82 by insulating bushings 90, 92, and 94,96, respectively. Those bushings are configured to extend around thearbor, to receive the bearings, and to hold the arbor and wheel awayfrom the bearings and housing so there is no metal-to-metal contactbetween the bearings/housing and the wheel/arbor. The bushings may bemade from many different insulating materials, such as PET-P or someother hard plastic. Bushings 90 and 92 are held in place between wheelhub 70 and an enlarged portion 100 on the arbor that has a greaterdiameter than the rest of the arbor. Bushings 94 and 96, in turn, arepositioned between enlarged portion 100 and a snap ring 102 on thearbor. In this manner, wheel 54 is supported by housing 50 forrotational movement, but is also electrically isolated from the housing.Bushing 90 includes a flange 91 sandwiched between hub 70 and bearing 80to prevent the hub from touching the bearing. Similarly, bushing 92includes a flange 93, and bushing 94 includes a flange 95, preventingenlarged portion 100 from touching either of bearings 80 or 82, andbushing 96 includes a flange 97 preventing snap ring 102 from touchingbearing 82. A pulley 84 is mounted on the end of arbor 72 opposite wheel54, and a belt (not shown) driven by motor assembly 16 may be used todrive pulley 84 and thereby spin arbor 72 and wheel 54 in bearings 80and 82 to move blade 14. The belt is typically non-conducting and thusdoes not electrically couple the arbor to the housing.

A cylindrical, insulating sleeve 110 is positioned and securely heldaround enlarged portion 100 by housing 50. Sleeve 110 may be press-fitinto an appropriate receptacle on the housing. Two electricallyconductive plates or tubes 112 and 114, having an outer diameter thatfits snugly within sleeve 110, are, in turn, press-fit into sleeve 110.Alternatively or additionally, plates 112 and 114 may be glued orotherwise mounted in sleeve 110. Sleeve 110 and plates 112 and 114 arecoaxial and concentric to enlarged portion 100 of arbor 72. Plates 112and 114 also have an inner diameter slightly larger than the diameter ofenlarged portion 100 so that they do not contact any part of arbor 72.Plates 112 and 114 are spaced apart in sleeve 110 by a gap 120. Plates112 and 114 may be made from any conductive material, such as brasstubing. Sleeve 110 protects plates 112 and 114 from damage and debris,and also electrically isolates the plates from housing 50.

Plates 112 and 114 may be thought of as contact detection plates thatare used to create capacitive couplings with the arbor and blade.Detection subsystem 22 includes suitable electrical circuitry (e.g.,such as described in U.S. Provisional Patent Application Ser. No.60/225,200, entitled “Contact Detection System for Power Equipment,”filed Aug. 14, 2000, by SD3, LLC, which is herein incorporated byreference) to transmit an input signal to plate 112, and to detect theinput signal through plate 114 via wires (not shown) attached to theplates, which wires may extend from the plates through a hole or holesin sleeve 110 to detection subsystem 22. In other words, detectionsubsystem 22 imparts a signal on plate 112. That signal then drives asignal onto arbor 72 by virtue of the capacitive coupling between theplate and the arbor. The arbor is conductively coupled to wheel 54, sothe signal induced on the arbor is also induced on the wheel. Blade 14loops around a significant portion of the perimeter of wheel 54, so thesignal on the wheel induces a signal on the blade. If wheel 54 includesa non-conductive, high-friction material such as rubber around itsperiphery to prevent the blade from slipping on the wheel when the wheelis rotated, then a signal is induced on the blade by a capacitivecoupling between the blade and the wheel. If blade 14 directly contactswheel 54, then the signal on the blade is the same as the signal on thewheel because of the conductive contact between the wheel and the blade.The signal on the arbor also induces a signal on plate 114 because ofthe proximity of the plate to the arbor. Thus, plate 114 monitors thesignal on the blade/arbor. When a person touches the blade, theeffective capacitance of the blade/arbor combination changes, causingthe signal on plate 114 to change, thereby signaling contact between theblade and a person.

Plates 112 and 114 are mounted close to, but spaced-apart from, arbor72. Those plates are capacitively coupled to the arbor by virtue oftheir size and placement parallel to and spaced-apart from the arbor. Itis within the scope of the present invention that the number, size andplacement of charge plates or tubes may vary.

The effect of this arrangement is to form two capacitors in seriesthrough the arbor, creating a capacitive shunt at the junction betweenthe capacitors. Plates or tubes 112 and 114 function as charge plates ofthe capacitors. The input signal is capacitively coupled from plates 112onto arbor 72, and then capacitively coupled from the arbor to plate114. Any change in the capacitance of the blade/arbor changes the signalcoupled to plate 114.

When a user touches blade 14, the capacitance of the user's body createsa capacitive load on the blade. As a result, the size of the capacitiveshunt between plates 112 and 114 and the blade is increased, therebyreducing the charge that reaches plate 114. Thus, the magnitude of theinput signal passed through the blade to plate 114 decreases when a usertouches the blade. Detection subsystem 22 is configured to detect thischange in the input signal and transmit a contact detection signal tocontrol subsystem 26.

In some cases, there may be a significant amount of resistance at thecontact point of the user's dry skin and the blade. This resistance mayreduce the capacitive coupling of the user's body to the blade. However,when the teeth on the blade penetrate the outer layer of the user'sskin, the moisture inherent in the internal tissue of skin will tend todecrease the resistance of the skin/blade contact, thereby establishinga solid electrical connection. The sensitivity of detection subsystem 22can be adjusted as desired to recognize even slight changes in the inputsignal.

Generally speaking, the spacing of the charge plates or tubes from thearbor is not critical, and may vary depending on the charge plate areaand the desired capacitive coupling.

Blade 14 must be electrically isolated from ground for the signal to beinduced on the blade. Additionally, capacitive couplings between theblade and other parts of the saw must be minimized so that the relativeincreased capacitance caused from a person touching the blade isreliably measurable. In other words, if the blade is capacitivelycoupled to other items, such as to a blade guard or to the housing, thenthe increased capacitance from a person touching the blade will beinsignificant compared to the combined capacitance of the blade andother items, meaning that the contact by the person will be harder todetect and the detection will be less reliable. Specifically, in a bandsaw, the blade will present a large surface area to wheel 52 andtherefore will capacitively couple to that wheel.

Band saw 10 addresses this issue by electrically isolating wheel 52 fromhousing 50, as shown in FIG. 4. Wheel 52 includes hub 121, and hub 121is mounted on spindle 122 by washer 124 and nut 126 threaded onto thespindle. The spindle is mounted to, or is part of, housing 50, so thespindle and housing support the wheel. Bearings 128 and 130 are pressfit into appropriate openings in the hub, and the bearings contactspindle 122 and support the hub and wheel on the spindle at spaced apartlocations, as shown. The bearings support the wheel for rotationalmovement on the spindle. Two non-conductive bushings 132 and 134, madefrom PET-P, a hard plastic, or some other non-conductive material, arepositioned between spindle 122 and bearings 128 and 130, respectively,to prevent the bearings from contacting the spindle. Bushing 132includes a flange 136 sandwiched between bearing 128 and washer 124 toprevent any metal-to-metal contact between the bearing and the washer orspindle. Bushing 134 includes a similar flange 138 sandwiched betweenbearing 130 and housing 50 to prevent metal-to-metal contact betweenthat bearing and the housing. With this construction, wheel 52 iselectrically isolated from housing 50.

Thus, in band saw 10, a charge or signal on plate 112 induces a chargeon arbor 72 and wheel 54, which in turn induces a charge on blade 14 andwheel 52. That charge then induces a signal on plate 114, which ismonitored by detection subsystem 22. When a person touches the blade,the effective capacitance of the blade/arbor/wheels combination changes,and that change is immediately detected by the detection subsystem. Nospecial or unique blade is required.

It will be appreciated that the size of charge plates 112 and 114 may beselected to provide a desired capacitance with the arbor. Indeed, thesize of the charge tubes may be different to provide differentcapacitances. For example, in the embodiment depicted in FIG. 3, chargeplate 112 is longer than charge plate 114, thereby providing a highercapacitance between charge plate 112 and the arbor, than between chargeplate 114 and the arbor. Alternatively, or additionally, the insidediameters of the charge tubes may be different to provide differentcapacitances due to different arbor-to-charge plate spacings.

It will be appreciated that while the charge plates or tubes andinsulating sleeve in the exemplary embodiment are cylindrical, othershapes may also be used. For example, insulating sleeve 110 may have arectangular outer cross-section while maintaining its circular innercross-section. Likewise, charge plates 112 and 114 may have any suitableouter cross-sectional shape to match the inner shape of the insulatingtube.

Since charge plates 112 and 114 should not come into contact with eachother, the fit between the charge plates and insulating sleeve 110 istypically tight enough to frictionally prevent movement of the chargeplates along the axis of the insulating sleeve. Alternatively, a bump orring may be formed or positioned on the inner diameter of the insulatingsleeve between the charge plates to prevent the charge plates fromcoming into contact. As a further alternative, caulk, glue, epoxy, orsimilar material may be applied between the charge plates and insulatingsleeve to prevent the charge plates from moving. As another alternative,one or more set-screws may be threaded through the insulating sleeve tobear against the charge tubes, making sure that the set screws do notcontact the housing or some other metal that would ground the chargeplates.

As explained above, blade 14 should be electrically isolated fromhousing 50, which is usually grounded. Thus, blade guide assemblies 60and 62, which may include ball-bearing guides and/or friction pads,etc., are constructed to electrically insulate the blade from the mainhousing.

Insulating sleeve 110 may also be constructed to receive a Hall Effector similar sensor to detect blade/arbor rotation, as described in moredetail in U.S. Provisional Patent Application Ser. No. 60/225,094,entitled “Motion Detection System for Use in Safety System for PowerEquipment,” filed Aug. 14, 2000, by SD3, LLC, which is herebyincorporated by reference.

Electrically isolating the blade as described above has the advantagethat the blade need not be capacitively isolated from wheels 52 and 54,which is difficult to do effectively. Nevertheless, and alternatively,capacitive couplings to the blade may be created in other ways, such asdisclosed in U.S. Provisional Patent Application Ser. No. 60/225,211,entitled “Apparatus and Method for Detecting Dangerous Conditions inPower Equipment,” filed Aug. 14, 2000, by SD3, LLC, and incorporatedherein by reference.

As explained above, when detection subsystem 22 detects contact betweenblade 14 and a person, reaction subsystem 24 reacts to prevent or limitinjury to the person. FIG. 5 shows schematically one embodiment of areaction system that stops the blade by cutting and gripping the blade.In the illustrated embodiment, reaction subsystem 24 is shown adjacentblade 14 and under table 58. Reaction subsystem 24 includes a backingplate 150 supported by the housing and positioned near one side of blade14. Backing plate 150 is made of either hardened or non-hardened metal.

A cutting pawl 152 is mounted adjacent backing plate 150 on the oppositeside of blade 14. Cutting pawl 152 is made from hardened steel. Cuttingpawl 152 is mounted to pivot in the direction of arrow 154 around pivotpin 156 mounted to the housing of the saw. Cutting pawl 152 includes acutting edge 158 on the end of the pawl opposite pivot pin 156. Pawl 152is configured to pivot down so that cutting edge 158 contacts blade 14and cuts the blade against backing plate 150. Cutting pawl 152 andbacking plate 150 may be thought of as brake mechanism 28 shown in FIG.1.

The force to pivot pawl 152 into the blade to cut the blade is, in part,provided by spring 160, which typically is a spring providingapproximately 10 to 500 pounds of force. The spring is configured toforce pawl 152 in the direction of arrow 154. When spring 160 pushescutting edge 158 into blade 14, the downward motion of the blade alsopushes pawl 152 downward, so that pawl 152 effectively locks on theblade and uses the motion of the blade to help cut the blade. Spring 160may be thought of as biasing mechanism 30 discussed above.

Cutting pawl 152 also includes a gripping surface 162 to grip the bladeand hold it against backing plate 150 both while the blade is cut andthereafter until the pawl is moved back away from the blade. Grippingsurface 162 may be simply a surface on the pawl, or it may be a layer ofhigh-friction material such as rubber or plastic, as shown in FIG. 5.Gripping surface 162 also may be thought of as part of brake mechanism28 discussed above. Gripping surface 162 is optional, and cutting pawl152 may be made without a gripping surface. In that case, reactionsubsystem 24 simply stops the blade by cutting it, withoutsimultaneously gripping the blade.

A fuse wire 164 is used to hold cutting pawl 152 away from blade 14until the detection subsystem detects that a person has contacted theblade. At that time, a firing subsystem 166 sends a surge of electricalcurrent through fuse wire 164, burning the wire and releasing thecutting pawl. Possible fuse wires and firing subsystems are disclosed inmore detail in U.S. Provisional Patent Application Ser. No. 60/225,056,entitled “Firing Subsystem for Use in a Fast-Acting Safety System,”filed Aug. 14, 2000, by SD3, LLC, and incorporated herein by reference.A mechanism providing mechanical advantage to hold the cutting pawl awayfrom the blade may be used, as described in U.S. Provisional PatentApplication Ser. No. 60/225,056, entitled “Spring-Biased Brake Mechanismfor Power Equipment,” filed Aug. 14, 2000, by SD3, LLC, and incorporatedherein by reference. Fuse wire 164 may be thought of as restrainingmechanism 32, and firing subsystem 166 may be thought of as releasemechanism 34.

When cutting pawl 152 cuts blade 14, the tension of the blade aroundwheels 52 and 54 is released and the blade stops immediately. The bladehas relatively little mass, and therefore little momentum, so the bladestops without incident. Additionally, the majority of blade 14 istypically within housing 50 so that the housing would contain the bladeeven if the blade tended to lash out when cut.

FIG. 6 shows another type of cutting pawl at 170. Cutting pawl 170 ispivotally mounted to cut blade 14, as described above. Cutting pawl 170includes a cutting edge 172 that extends helically away from the bladerelative to the blade so that the cutting edge first contacts the bladeat a point designated at 174, and then progressively moves into andacross the blade. The cutting edge may extend helically away from blade14, as if the pawl had been twisted around an axis perpendicular to thepivot axis of the pawl. Additionally, the pivot point of pawl 170 may bemounted to the housing of the saw so that the pawl pushes blade 14 back,away from where the blade would normally cut, thereby retracting orpushing the blade away from the point where a person most likely wouldaccidentally contact the blade. For example, the pivot may benon-parallel to the table so that the pawl pushes the blade down andback. FIG. 6 also shows a backing plate 176 against which cutting pawl170 cuts blade 14. Backing plate 176 includes a curved surface 178 thatfollows the radius of cutting pawl 170 as it pivots. Cutting pawl 170,shown in FIG. 6, may be released to cut blade 14 as described above inconnection with the embodiment shown in FIG. 5.

FIG. 7 shows a top view, and FIG. 8 shows a front view, of another wayof cutting blade 14 upon the detection of contact between a person andthe blade. The embodiment shown in FIGS. 7 and 8 includes two cuttingpawls, 180 and 182, each positioned on one side of blade 14, to act likescissors to cut the blade. FIG. 8 shows how the pawls are positionedvertically relative to each other to act like scissors. The pawls wouldbe released by a fuse wire and firing system, and pushed into the bladeby springs, as described above. The pawls may be configured to strikethe front of blade 14 first so that the pawls retract or push the bladeback and away from a user of the saw. Pawls 180 and 182 also could beconfigured and mounted to the housing to strike blade 14 at an angle sothat they lock onto the blade and so that they are self-feeding due tothe motion of the blade. Pawls 180 and 182 also could be mounted so thatthey move down with blade 14 as they cut the blade.

Additionally, any of the cutting pawls described above may have acutting edge made of carbide or hardened steel.

One example of an electronic subsystem 1000 of contact detectionsubsystem 22 according to the present invention is illustrated in moredetail in FIG. 9. Electronic subsystem 1000 is adapted to work with thetwo-plate capacitive coupling system described in U.S. ProvisionalPatent Application Ser. No. 60/225,211, entitled “Apparatus and Methodfor Detecting Dangerous Conditions in Power Equipment,” filed Aug. 14,2000. Electronic subsystem 1000 includes an excitation system 1010 and amonitoring or contact sensing system 1020. However, it will beappreciated by those of skill in the electrical arts that the exemplaryconfiguration of electronic subsystem 1000 illustrated in FIG. 9 is justone of many configurations which may be used. Thus, it will beunderstood that any suitable embodiment or configuration could be usedwithin the scope of the invention.

As shown in FIG. 9, excitation system 1010 includes an oscillatorcircuit that generates a wave input signal, such as a square wavesignal, at a frequency of approximately 200 khz and voltage amplitude of12 volts. Alternatively, excitation system 1010 may be configured togenerate a signal of a different frequency and/or a different amplitudeand/or different waveform. The oscillator is formed by a pair ofinverters 1030, 1040 from a CD4040 configured as a bistable oscillator.The output of inverter 1030 is connected to a 100 pF capacitor 1050,which is connected through a 100 kΩ resistor 1060 to the input ofinverter 1040. A 10 kΩ resistor 1070 is connected between the output ofinverter 1040 to the junction between capacitor 1050 and resistor 1060.The output of inverter 1040 is connected to the input of inverter 1030.A 10 kΩ resistor 1080 connects the output of inverter 1030 to the inputof another inverter 1090, which serves as an output buffer to drive theinput wave signal onto the blade. A 2 kΩ series resistor 1100 functionsto reduce any ringing in the input signal by damping the high frequencycomponents of the signal.

It will be appreciated that the particular form of the oscillator signalmay vary and there are many suitable waveforms and frequencies that maybe utilized. The waveform may be chosen to maximize the signal-to-noiseratio, for example, by selecting a frequency at which the human body hasthe lowest resistance or highest capacitance relative to the workpiecebeing cut. As an additional variation, the signal can be made asymmetricto take advantage of potentially larger distinctions between theelectrical properties human bodies and green wood at high frequencywithout substantially increasing the radio-frequency power radiated. Forinstance, utilizing a square wave with a 250 khz frequency, but a dutycycle of five percent, results in a signal with ten times higherfrequency behavior than the base frequency, without increasing theradio-frequency energy radiation. In addition, there are many differentoscillator circuits that are well known in the art and which would alsobe suitable for generating the excitation signal.

The input signal generated by the oscillator is fed through a shieldedcable 1110 onto charge plate 440. Shielded cable 1110 functions toinsulate the input signal from any electrical noise present in theoperating environment, insuring that a “clean” input signal istransmitted onto charge plate 440. Also, the shielded cable reducescross talk between the drive signal and the detected signal that mightotherwise occur should the cables run close together. Alternatively,other methods may be used to prevent noise in the input signal. As afurther alternative, monitoring system 1020 may include a filter toremove any noise in the input signal or other electrical noise detectedby charge plate 460. Shielded cable 1110 also reduces radio-frequencyemissions relative to an unshielded cable.

As described in more detail in U.S. Provisional Patent Application Ser.No. 60/225,211, entitled “Apparatus and Method for Detecting DangerousConditions in Power Equipment,” filed Aug. 14, 2000, the input signal iscoupled from charge plate 440 to charge plate 460 via blade 400. Asshown in FIG. 9, the signal received on charge plate 460 is then fed viaa shielded cable 1120 to monitoring system 1020. The monitoring systemis configured to detect a change in the signal due to contact betweenthe user's body and the blade. It will be appreciated that monitoringsystem 1020 may be implemented in any of a wide variety of designs andconfigurations. In the exemplary embodiment depicted in FIG. 9,monitoring system 1020 compares the amplitude of the input signalreceived at charge plate 460 to a determined reference voltage. In theevent that the input signal received at charge plate 460 falls below thereference voltage for a determined time, the monitoring system producesan output signal to reaction subsystem 24. The reaction subsystem isconfigured to receive the output signal and immediately act to stop theblade.

The particular components of monitoring system 1020 may vary dependingon a variety of factors including the application, the desiredsensitivity, availability of components, type of electrical poweravailable, etc. In the exemplary embodiment, a shielded cable 1120 isconnected between charge plate 460 and a voltage divider 1130. Voltagedivider 1130 is formed by two 1MΩ resistors 1140, 1150 connected inseries between the supply voltage (typically about 12 volts) and ground.The voltage divider functions to bias the output signal from chargeplate 460 to an average level of half of the supply voltage. The biasedsignal is fed to the positive input of an op-amp 1160. Op-amp 1160 maybe any one of many suitable op-amps that are well known in the art. Anexample of such an op-amp is a TL082 op-amp. The negative input of theop-amp is fed by a reference voltage source 1170. In the exemplaryembodiment, the reference voltage source is formed by a 10 kΩpotentiometer 1180 coupled in series between two 10 kΩ resistors 1190,1200, which are connected to ground and the supply voltage,respectively. A 0.47 μF capacitor 1210 stabilizes the output of thereference voltage.

As will be understood by those of skill in the art, op-amp 1160functions as a comparator of the input signal and the reference voltage.Typically, the voltage reference is adjusted so that its value isslightly less than the maximum input signal voltage from charge plate460. As a result, the output of the op-amp is low when the signalvoltage from the charge plate is less than the reference voltage andhigh when the signal voltage from the charge plate is greater than thereference voltage. Where the input signal is a periodic signal such asthe square wave generated by excitation system 1010, the output ofop-amp 1160 will be a similar periodic signal. However, when a usercontacts the blade, the maximum input signal voltage decreases below thereference voltage and the op-amp output no longer goes high.

The output of op-amp 1160 is coupled to a charging circuit 1220.Charging circuit 1220 includes a 240 pF capacitor 1230 that is connectedbetween the output of op-amp 1160 and ground. A 100 kΩ dischargeresistor 1240 is connected in parallel to capacitor 1230. When theoutput of op-amp 1160 is high, capacitor 1230 is charged. Conversely,when the output of op-amp 1160 is low, the charge from capacitor 1230discharges through resistor 1240 with a time constant of approximately24 μs. Thus, the voltage on capacitor 1230 will discharge to less thanhalf the supply voltage in approximately 25-50 μs unless the capacitoris recharged by pulses from the op-amp. A diode 1250 prevents thecapacitor from discharging into op-amp 1160. Diode 1250 may be any oneof many suitable diodes that are well known in the art, such as a 1N914diode. It will be appreciated that the time required for capacitor 1230to discharge may be adjusted by selecting a different value capacitor ora different value resistor 1240. As described above, charging circuit1220 will be recharged repeatedly and the voltage across capacitor 1230will remain high so long as the detected signal is receivedsubstantially unattenuated from its reference voltage at op-amp 1160.The voltage from capacitor 1230 is applied to the negative input of anop-amp 1260. Op-amp 1260 may be any one of many suitable op-amps, whichare well known in the art, such as a TL082 op-amp. The positive input ofop-amp 1260 is tied to a reference voltage, which is approximately equalto one-half of the supply voltage. In the exemplary embodiment depictedin FIG. 9, the reference voltage is provided by reference voltage source1170.

So long as charging circuit 1220 is recharged, the output of op-amp 1260will be low. However, if the output of op-amp 1160 does not go high fora period of 25-50 μs, the voltage across capacitor 1230 will decay toless than the reference voltage, and op-amp 1260 will output a highsignal indicating contact between the user's body and the blade. Asdescribed in U.S. Provisional Patent Application Ser. No. 60/225,056,entitled “Firing Subsystem for Use in a Fast-Acting Safety System,” U.S.Provisional Patent Application Ser. No. 60/225,170, entitled“Spring-Biased Brake Mechanism For Power Equipment,” and U.S.Provisional Patent Application Ser. No. 60/225,169, entitled “BrakeMechanism for Power Equipment,” all filed Aug. 14, 2000, the outputsignal from op-amp 1260 is coupled to actuate reaction subsystem 24 andstop the blade. The time between contact and activation of the reactionsystem can be adjusted by selecting the time constant of capacitor 1230and resistor 1240.

It should be noted that, depending on the size, configuration and numberof teeth on the blade and the position of contact with the operator, theelectrical contact between the operator and blade will often beintermittent. As a result, it is desirable that the system detectcontact in a period less than or equal to the time a single tooth wouldbe in contact with a user's finger or other body portion. For example,assuming a 10-inch circular blade rotating at 4000 rpm and a contactdistance of about one-quarter of an inch (the approximate width of afingertip), a point on the surface of the blade, such as the point of atooth, will be in contact with the user for approximately 100 μs. Afterthis period of contact, there will normally be an interval of no contactuntil the next tooth reaches the finger. The length of the contact andnon-contact periods will depend on such factors as the number of teethon the blade and the speed of rotation of the blade.

It is preferable, though not necessary, to detect the contact with thefirst tooth because the interval to the second tooth may be substantialwith blades that have relatively few teeth. Furthermore, any delay indetection increases the depth of cut that the operator will suffer.Thus, in the exemplary embodiment, the charging circuit is configured todecay within approximately 25-50 μs to ensure that monitoring system 102responds to even momentary contact between the user's body and theblade. Further, the oscillator is configured to create a 200 khz signalwith pulses approximately every 5 μs. As a result, several pulses of theinput signal occur during each period of contact, thereby increasing thereliability of contact detection. Alternatively, the oscillator andcharging circuit may be configured to cause the detection system torespond more quickly or more slowly. Generally, it is desirable tomaximize the reliability of the contact detection, while minimizing thelikelihood of erroneous detections.

As described above, the contact between a user's body and the teeth ofblade 400 might be intermittent depending on the size and arrangement ofthe teeth. Although monitoring system 1020 typically is configured todetect contact periods as short as 25-50 μs, once the first tooth of theblade passes by the user's body, the contact signal received by thesecond electrical circuit may return to normal until the next toothcontacts the user's body. As a result, while the output signal at op-amp1260 will go high as a result of the first contact, the output signalmay return low once the first contact ends. As a result, the outputsignal may not remain high long enough to activate the reaction system.For instance, if the output signal does not remain high long enough toactuate firing subsystem 760, fusible member 700, may not melt.Therefore, monitoring system 1020 may include a pulse extender in theform of charging circuit 1270 on the output of op-amp 1260, similar tocharging circuit 1220. Once op-amp 1260 produces a high output signal,charging circuit 1270 functions to ensure that the output signal remainshigh long enough to sufficiently discharge the charge storage devices tomelt the fusible member. In the exemplary embodiment, charging circuit1270 includes a 0.47 μF capacitor 1280 connected between the output ofop-amp 1260 and ground. When the output of op-amp 1260 goes high,capacitor 1280 charges to the output signal level. If the output ofop-amp 1260 returns low, the voltage across capacitor 1280 dischargesthrough 10 k resistor 1290 with a time constant of approximately 4.7 ms.A diode 1300, such as an 1N914 diode, prevents capacitor 1280 fromdischarging through op-amp 1260. The pulse extender insures that even ashort contact with a single tooth will result in activation of thereaction system.

The above-described system is capable of detecting contact withinapproximately 50 μs and activating the reaction system. As described inmore detail in U.S. Provisional Patent Application Ser. No. 60/225,056,entitled “Firing Subsystem for Use in a Fast-Acting Safety System,” U.S.Provisional Patent Application Ser. No. 60/225,170, entitled“Spring-Biased Brake Mechanism For Power Equipment,” and U.S.Provisional Patent Application Ser. No. 60/225,169, entitled “BrakeMechanism for Power Equipment,” all filed Aug. 14, 2000, in the contextof reaction system for braking a saw blade, a brake can be released inapproximately less than 100 μs and as little as 20 μs. The brakecontacts the blade in approximately one to approximately threemilliseconds. The blade will normally come to rest within not more than2-10 ms of brake engagement. As a result, injury to the operator isminimized in the event of accidental contact with the cutting tool. Withappropriate selection of components, it may be possible to stop theblade within 2 ms, or less.

While exemplary embodiments of excitation system 1010 and monitoringsystem 1020 have been described above with specific components havingspecific values and arranged in a specific configuration, it will beappreciated that these systems may be constructed with many differentconfigurations, components, and values as necessary or desired for aparticular application. The above configurations, components, and valuesare presented only to describe one particular embodiment that has proveneffective, and should be viewed as illustrating, rather than limiting,the invention.

FIG. 10 shows alternative embodiments of excitation system 1010 andmonitoring system 1020, as well as firing system 760, which is describedin more detail in U.S. Provisional Patent Application Ser. No.60/225,056, titled “Firing Subsystem for Use in a Fast-Acting SafetySystem,” filed Aug. 14, 2000. Alternative excitation system 1010 isconfigured to generate a square wave signal using only a singlecomparator 1330 such as an LM393 comparator. A 1M resistor 1340 isconnected between the high input terminal of comparator 1330 and ground.Another 1M resistor 1350 is connected between the high input terminal ofcomparator 1330 and a low voltage supply V. A 1M resistor 1360 isconnected between the high input terminal of the comparator and theoutput of the comparator. A 100 pF capacitor 1370 is connected betweenthe low input terminal of the comparator and ground. A 27 k resistor1380 is connected between the low input terminal of the comparator andthe output of the comparator. A 3.3 k resistor 1390 is connected betweenthe low voltage supply V and the output of the comparator. Thealternative oscillator circuit illustrated in FIG. 12 produces a squarewave having a frequency of approximately 3-500 khz. A 1 k resistor 1400is connected between the output of the comparator and shielded cable1110 to reduce ringing. It will be appreciated that the values of one ormore elements of alternative excitation system 1010 may be varied toproduce a signal having a different frequency, waveform, etc.

As in the exemplary embodiment described above, the signal generated byalternative excitation system 1010 is fed through shielded cable 1110 tocharge plate 440. The signal is capacitively coupled to charge plate 460via blade 400. Alternative monitoring system 1020 receives the signalfrom charge plate 460 via shielded cable 1120 and compares the signal toa reference voltage. If the signal falls below the reference voltage forapproximately 25 μs, an output signal is generated indicating contactbetween the blade and the user's body.

Alternative monitoring system 1020 includes a voltage divider 1130,which is formed of 22k resistors 1410 and 1420. The voltage dividerbiases the signal received via cable 1120 to half the low voltage supplyV. The lower resistance of resistors 1410, 1420 relative to resistors114, 1150 serves to reduce 60 hz noise because low-frequency signals areattenuated. The biased signal is fed to the negative input terminal of asecond comparator 1430, such as an LM393 comparator. The positiveterminal of comparator 1430 is connected to reference voltage source1440. In the depicted embodiment, the reference voltage source is formedby a 10 kΩ potentiometer 1450 coupled in series between two 100 kΩresistors 1460, 1470 connected to the low voltage supply V and ground,respectively. A 0.1 μf capacitor 1480 stabilizes the output of thereference voltage. As before, the reference voltage is used to adjustthe trigger point.

The output of second comparator 1430 is connected to the base terminalof an NPN bipolar junction transistor 1490, such as a 2N3904 transistor.The base terminal of transistor 1490 is also connected to low voltagesupply V through a 100 k resistor 1500, and to ground through a 220 pFcapacitor 1510. Potentiometer 1450 is adjusted so that the voltage atthe positive terminal of comparator 1430 is slightly lower than the highpeak of the signal received at the negative terminal of the secondcomparator when there is no contact between the blade and the user'sbody. Thus, each high cycle of the signal causes the second comparatoroutput to go low, discharging capacitor 1510. So long as there is nocontact between the blade and the user's body, the output of the secondcomparator continues to go low, preventing capacitor 1510 from chargingup through resistor 1500 and switching transistor 1490 on. However, whenthe user's body contacts the blade or other isolated element, the signalreceived at the negative terminal of the second comparator remains belowthe reference voltage at the positive terminal and the output of thesecond comparator remains high. As a result, capacitor 1510 is able tocharge up through resistor 1500 and switch transistor 1490 on.

The collector terminal of transistor 1490 is connected to low voltagesupply V, while the emitter terminal is connected to 680Ω resistor 1520.When transistor 1490 is switched on, it supplies an output signalthrough resistor 1520 of approximately 40 mA, which is fed toalternative firing system 760. As described in more detail in U.S.Provisional Patent Application Ser. No. 60/225,056, titled “FiringSubsystem for Use in a Fast-Acting Safety System,” filed Aug. 14, 2000,the alternative firing circuit includes fusible member 700 connectedbetween a high voltage supply HV and an SCR 6130, such as an NTE 5552SCR. The gate terminal of the SCR is connected to resistor 1520. Thus,when transistor 1490 is switched on, the approximately 40 mA currentthrough resistor 1520 turns on SCR 6130, allowing the high voltagesupply HV to discharge to ground through fusible member 700. Once theSCR is switched on, it will continue to conduct as long as the currentthrough fusible member 700 remains above the holding current ofapproximately 40 mA, even if the current to the gate terminal isremoved. Thus, the SCR will conduct current through the fusible memberuntil the fusible member is melted or the high voltage source isexhausted or removed. The fact that the SCR stays on once triggeredallows it to respond to even a short pulse through resistor 1520.

FIG. 10 also illustrates an exemplary electrical supply system 1540configured to provide both low voltage supply V and high voltage supplyHV from standard 120 VAC line voltage. Electrical supply system 1540 isconnected to provide low voltage supply V and high voltage supply HV toalternative excitation system 1010, alternative monitoring system 1020,and alternative firing system 760. The line voltage is connected througha 100Ω resistor 1550 and a diode 1560, such as a 1N4002 diode, to a 1000μF charge storage capacitor 1570. The diode passes only the positiveportion of the line voltage, thereby charging capacitor 1570 toapproximately 160V relative to ground. The positive terminal ofcapacitor 1570 serves as the high voltage supply HV connected to fusiblelink 700. When SCR 6130 is switched on upon detection of contact betweenthe blade and the user's body, the charge stored in capacitor 1570 isdischarged through the fusible link until it melts. It will beappreciated that the size of capacitor 1570 may be varied as required tosupply the necessary current to melt fusible member 700. As described inU.S. Provisional Patent Application Ser. No. 60/225,056, titled “FiringSubsystem for Use in a Fast-Acting Safety System,” filed Aug. 14, 2000,use of a HV capacitor leads to a much higher current surge, andtherefore a faster melting of the fusible member than is the case with alow voltage system.

The positive terminal of capacitor 1570 also provides a transformer-lesssource of voltage for low voltage supply V, which includes a 12 kresistor 1580 connected between the positive terminal of capacitor 1570and a reverse 40V Zener diode 1590. Diode 1590 functions to maintain arelatively constant 40V potential at the junction between the diode andresistor 1580. It can be seen that the current through the 12 k resistorwill be about 10 mA. Most of this current is used by the low voltagecircuit, which has a relatively constant current demand of about 8 mA.Note that while resistor 1580 and diode 1590 discharge some current fromcapacitor 1570, the line voltage supply continuously recharges thecapacitor to maintain the HV supply. A 0.1 μF capacitor 1600 isconnected in parallel with diode 1590 to buffer the 40V potential of thediode, which is then connected to the input terminal of an adjustablevoltage regulator 1610, such as an LM317 voltage regulator. The ratio ofa 1 k resistor 1620 connected between the output terminal and adjustmentterminal, and a 22 k resistor 1630 connected between the adjustmentterminal and ground, set the output voltage of regulator 1610 toapproximately 30VDC. A 50 μF capacitor 1640 is connected to the outputterminal of regulator 1610 to buffer sufficient charge to ensure thatlow voltage supply V can provide the brief 40 mA pulse necessary toswitch on SCR 6130. The described low voltage source is advantageousbecause of its low cost and low complexity.

It should be noted that when high voltage supply HV is dischargedthrough fusible member 700, the input voltage to voltage regulator 1610may temporarily drop below 30V, thereby causing a corresponding drop inthe low voltage supply V. However, since the reaction system has alreadybeen triggered, it is no longer necessary for the detection system tocontinue to function as described and any drop in low voltage supply Vwill not impair the functioning of safety system 18.

It will be appreciated by those of skill in the electrical arts that thealternative embodiments of excitation system 1010, monitoring system1020, firing system 760, and electrical supply system 1540 may beimplemented on a single substrate and/or in a single package.Additionally, the particular values for the various electrical circuitelements described above may be varied depending on the application.

One limitation of the monitoring systems of FIGS. 9 and 10 is that theyactuate the reaction system whenever the incoming amplitude from chargeplate 460 drops below a preset threshold. Under most circumstances thisrepresents a reliable triggering mechanism. However, when cutting greenwood, a substantial additional capacitive and resistive load is coupledto the blade. The moisture in green wood gives it a very high dielectricconstant, and an increased conductivity relative to dry wood. In fact,when cutting very green wood, i.e. over 50% moisture content, theamplitude of the signal on charge plate 460 can drop to a levelequivalent to what is seen when a user contacts the blade. Thus, thesystems of FIGS. 9 and 10 are limited in their ability to offerprotection while processing green wood.

Another embodiment of an electronic subsystem 100 adapted to accommodategreen wood and offering certain other benefits is shown in FIGS. 11-19.As shown in FIG. 11, system 1000 includes an excitation system 1010 inthe form of a class-C amplifier connected to a micro-controller 1710.System 1000 also includes a monitoring system 1020 in the form of acontact sense circuit connected to controller 1710. A power supply 1730supplies power to the various elements of system 1000. A motorcontroller 1740 is adapted to turn a motor off and on based on signalsfrom the controller. A boost regulator 1750 operates to charge a firingsystem 1760. A rotation sense circuit 1770 detects rotation of thecutting tool. Lastly, a user interface 1780 is provided to allow a userto control operation of the saw and provide feedback on the status ofthe system.

A computer program listing for a controller as shown in FIG. 5 issubmitted herewith as a Computer Program Listing Appendix.

FIG. 12 illustrates the circuitry of the class-C amplifier in moredetail. The amplifier includes a drive output that is coupled to plate440 as shown in FIG. 11. The drive output is sinusoidal at about 500 khzand the amplitude is adjustable between about 3 volts and 25 volts. A32-volt input supply line from the power supply provides power for theamplifier. The base frequency is provided by a 500 khz square wave inputfrom the controller. The amplitude is controlled by pulse widthmodulation from the controller.

The controller is programmed to adjust the drive voltage output from theamplifier to maintain a predetermined amplitude at plate 460 undervarying capacitive loads. Thus, when cutting green wood, the controllerramps up the drive voltage to maintain the desired voltage on plate 460.The controller is preferably capable of skewing the drive voltagebetween about 1 and 50% per millisecond, and more preferably between 1and 10%. This allows the system to maintain a constant output levelunder the varying load created while sawing green wood, or such as mightbe created by placing a conductive member such a fence near the blade.The controller should preferably not skew the drive voltage by much morethan 50% per millisecond, or it may counteract the drop in signalcreated by a user contact event.

FIG. 13 illustrates the change in signal amplitude seen at plate 460 asthe teeth of a 10-inch, 36-tooth saw blade spinning at 4000 rpm contactsa user's finger. Each of the drops in the signal amplitude is from asingle tooth moving through the skin of the finger. It can be seen, forinstance, that the signal amplitude drops by about 30% over about 500 asthe second tooth strikes the finger. When cutting very green wood, thesignal attenuation upon contact will be more like 15%, but will occurover the same 500. Therefore, as long as the system can detect a contactevent of a 5-25% or greater drop in less than 1000, providing a skewrate of around 10% per millisecond should not override an actual event.It will be understood that the skew rate and trigger thresholds can beadjusted as desired. The primary limiting factor is that the triggerthreshold should not be so small that noise creates false triggers,unless false triggers are acceptable.

FIG. 14 shows the details of the contact sense circuit. The contactsense circuit receives input from plate 460. In this embodiment, thepreferred capacitive coupling between the blade and the plates is about30 pF for the drive plate and about 10 pF for plate 460. The largerdrive plate size improved signal transfer for a given total capacitanceof both plates. The actual values are not critical, and equal valuescould be used as well. Generally speaking, the capacitance of the driveplate should be comparable to the human body capacitance to be detected,i.e. 10-200 pF.

The input from plate 460 is fed through a high-pass filter 1790 toattenuate any low frequency noise, such as 60 hz noise, picked up byplate 460. Filter 1790 can also provide amplification of the signal to adesired level as necessary. The output of the filter is fed into a setof comparators 1800, 1810. Comparator 1800 pulses high briefly if themaximum signal amplitude from the filter exceeds the value at itspositive input set by voltage divider 1820. The output pulses from thecomparator are fed to the controller. The controller samples over a 2000window and modulates the drive amplitude to attempt to maintain thesensed voltage at a level so that 50% of the waveform cycles generate apulse through comparator 1800. If less than 50% generate pulses, thenthe controller raises the drive voltage by a set amount. Likewise, ifmore than 50% generate pulses, the drive voltage is lowered. The systemcan be configured to step by larger or smaller amounts depending on thedeviation from 50% observed during a particular window. For instance, if45 pulses are observed, the system may step up the drive amplitude by1%. However, if only 35 pulses are observed, the system may step by 5%.The system will continually “hunt” to maintain the proper drive level.By selecting the window duration and adjustment amount, it is possibleto control the skew rate to the desired level as described above.

Comparator 1810 pulses every cycle of the waveform so long as the sensedvoltage exceeds a lower trigger threshold set by voltage divider 1820.Therefore, under normal circumstances, this is a 500 khz pulse. Thepulse output from comparator 1810 is fed through a divide-by-fourcircuit formed by two D-flip flops to reduce the frequency to 125 khz—oran 8 μS period. The output of the divider is fed to the controller. Thecontroller monitors this line to insure that a pulse occurs at leastevery 180. Therefore, if more than about half of the pulse are missingin over an 18 μS period, the controller will trigger the reactionsystem. Of course, the particular period can be selected as desired tomaximize reliability of contact detection and minimize false triggers. Abenefit of the described arrangement is that a single pulse or even twomay be missing, such as due to noise, without triggering the system.However, if more pulses are missing, the system will still be triggeredreliably. The particular trigger level for missing pulses is set by thevoltage divider. This level will typically be between 5 and 40% for thedescribed system.

FIG. 15 illustrates the circuit of power supply 1730. The power supplyincludes an unregulated 32-volt output and regulated 5, 15 and 24-voltoutputs. The 24-volt output is used to power the excitation signal,which has a relatively large voltage, and the 32-volt output powers acapacitor charging circuit described below. The 5-volt output powers thecontroller and other logic circuitry, while the 15-volt output operatesmost of the analog electronics. A low-voltage output is monitored by thecontroller to insure that adequate voltage is present to operate thesystem.

Boost regulator 1750 and firing system 1760 are shown in FIG. 16. Boostregulator 1750 includes a buck-boost charger 1830 that steps up the32-volt supply input to 180 volts for charging the firing circuit. Thecontroller provides a 125 khz input to modulate the buck-boost cycle ofthe charger. A regulator circuit 1840 monitors the voltage from thefiring circuit and turns the charger on or off as necessary to maintainthe charge near 180 volts. The regulator circuit is constructed with apredetermined amount of hysteresis so that the charger will turn on whenthe firing circuit voltage falls below 177 volts and turn off when thevoltage reaches 180 volts, as set by the voltage divider inputs andfeedback to comparator 1850. The output of comparator 1850 is fed to thecontroller. By monitoring the charge and discharge time based on thestate of the output of comparator 1850, the controller can verify thatthe capacitor in the firing circuit is operating properly and storingadequate charge. An overvoltage circuit uses a 220V transient suppressorto signal the controller if the voltage on the capacitor exceeds about220V. This testing is described in more detail in U.S. ProvisionalPatent Application Ser. No. 60/225,059, titled “Logic Control forFast-Acting Safety System,” filed Aug. 14, 2000. The firing circuit isdescribed in more detail in U.S. Provisional Patent Application Ser. No.60/225,056, titled “Firing Subsystem for Use in a Fast-Acting SafetySystem,” filed Aug. 14, 2000.

FIG. 17 illustrates the circuitry of motor control 1740. The motorcontrol receives a logic level control signal from the controller toturn the motor on and off based on input from the user interface,described in more detail below. The motor control also turns off themotor when a trigger event occurs. The logic signal is electricallyisolated from the motor voltage by an optoisolated triac driver. Thisisolates the ground of the detection system from the ground of the motorpower. A mechanical relay or similar device can also be used and willprovide the same isolation. When the optoisolated triac drive receives asignal from the controller, it turns on Q6040K7 triac to provide powerto the machine.

The rotation sense circuit is shown in FIG. 18. The purpose of therotation sense circuit is to insure that the contact detection system isnot turned off until the cutter or blade as stopped. The rotation sensecircuit utilizes a hall-effect sensor that is located adjacent arotating portion of the machine. A small magnet is inserted in therotating portion to signal the hall-effect sensor. Output of thehall-effect sensor is fed to the controller. As described in more detailin U.S. Provisional Patent Application Ser. No. 60/225,059, titled“Logic Control for Fast-Acting Safety System,” filed Aug. 14, 2000, thecontroller monitors the output of the hall-effect sensor to determinewhen the cutter has coasted to a stop. Once the cutter stops, any sensedcontact will no longer trigger the reaction system. It should be notedthat rotation of the cutter could be detected by other arrangements aswell. Various suitable mechanisms are described in U.S. ProvisionalPatent Application Ser. No. 60/225,094, titled “Motion Detecting Systemfor Use in A Safety System for Power Equipment,” filed Aug. 14, 2000.

For instance, a small eccentricity can be placed on the cutter or someother isolated structure that rotates with the cutter, such as thearbor. This eccentricity can be placed to pass by sense plate 460 or bya separate sensing plate. The eccentricity will modulate the detectedsignal amplitude so long as the cutter is rotating. This modulation canbe monitored to detect rotation. If the eccentricity is sensed by senseplate 460, it should be small enough that the signal modulationgenerated will not register as a contact event. As another alternative,rotation can be sensed by electromagnetic feedback from the motor.

The controller may also be designed to monitor line voltage to insurethat adequate voltage is present to operate the system. For instance,during motor start up, the AC voltage available to the safety system maydrop nearly in half depending on the cabling to the saw. If the voltagedrops below a safe level, the controller can shut off the saw motor.Alternatively, the controller may include a capacitor of sufficientcapacity to operate the system for several seconds without power inputwhile the saw is starting.

User interface 1780 is shown in FIG. 19. The user interface includesstart, stop and bypass buttons that are used to control the operation ofthe saw. The bypass button allows the user to disable the contactdetection system for a single on/off cycle of the saw so as to be ableto saw metal or other materials that would otherwise trigger thereaction system. The user interface also includes red and green LEDsthat are used to report the status of the system to a user. More detailson the operation of suitable user interfaces are described in U.S.Provisional Patent Application Ser. No. 60/225,059, titled “LogicControl for Fast-Acting Safety System,” filed Aug. 14, 2000.

Two additional electronic configurations for detection subsystem 22 areshown in FIGS. 20-24. As illustrated in FIG. 21, the alternativedetection systems utilize a micro-controller 1710 to manage and monitorvarious functions. An excitation system delivers a 350 khz sine wavedrive signal through plate 44 to the blade. The circuit for generatingthe drive signal is illustrated in FIG. 21. The excitation circuit usesa 700 khz oscillator with an output fed into a double to generate a 1.4Mhz signal. The output of the double is fed into a set of S-R flip-flopsto extract phase signals at 90-degree intervals. The phase signals areused to drive a synchronous detection system that forms one of the twoembodiments of FIGS. 20-24 and is shown in more detail in FIG. 23. The350 khz square wave 180-degree phase signal is fed through an inverterand a buffer amplifier into a Q=10, 350 khz band pass filter.

The output of the band pass filter is a 350 khz sine wave that is fedthrough another buffer amplifier to a sense amplifier 190 shown in FIG.22. The output of the sense amplifier is fed to plate 440 and the inputfrom plate 460 is fed back to the negative input. When a user touchescutter 400, the feedback on the sense amplifier is reduced, therebycausing the output amplitude to go up. The result of this arrangement isthat the drive amplitude on the blade is small during normal use andrises only when a user touches the blade or green wood is cut. In thisembodiment, the preferred capacitive coupling of the plates to the bladeis about 90 pF each, although other values could be used.

The output of the sense amplifier is fed through a buffer and into a 350khz band pass filter to filter out any noise that may have been pickedup from the blade or plates. The output of the band pass filter is fedthrough a buffer and into a level detector. The level detector generatesa DC output proportional to the amplitude of the sense amplifier. Theoutput of the level detector is smoothed by an RC circuit to reduceripple and fed into a differentiator. The differentiator generates anoutput proportional to the rate of change of the sense amplifier outputamplitude. As mentioned above, the sense amplifier output only changeswhen a user touches the blade or green wood is cut. The change whencutting green wood is slow relative to what happens when a user touchesthe blade. Therefore, the differentiator is tuned to respond to a usercontact, while generating minimal response to green wood. The output ofthe differentiator is then fed to a comparator that acts as thresholddetector to determine if the output of the differentiator has reached apredetermined level set by the voltage divider network. The output ofthe threshold detector is fed through a Schmitt-trigger that signals thecontroller that a contact event has occurred. An RC network acts as apulse stretcher to insure that the signal lasts long enough to bedetected by the controller.

The output from the level detector is also fed to an analog to digitalinput on the controller. It may be that the under some circumstances,such as while cutting extremely green wood, the response of the senseamplifier will be near saturation. If this happens, the amplifier may nolonger be capable of responding to a contact event. In order to providea warning of this situation, the controller monitors this line to makesure that the detected level stays low enough to allow a subsequentcontact to be detected. If an excess impedance load is detected, thecontroller can shut down the saw without triggering the reaction systemto provide the user with a warning. If the user wants to continue, theycan initiate the bypass mode as described above.

The second of the two alternative detection systems of FIGS. 20-24 is asynchronous detector that uses the phase information generated by theflip-flops in FIG. 21. This system drives plate 44 through the ALT DRIVEcircuit shown in FIG. 21. This ALT DRIVE circuit and the detectioncircuit of FIG. 23 are substituted for the circuit of FIG. 22. As shownin FIG. 23, the signal from plate 460 is fed through a pair ofbuffer/amplifiers into a set of analog switches. The switches arecontrolled by the phase information from the flip-flops. Thisarrangement generates an output signal that is proportional to theamplitude of the signal detected from plate 460 with improved noiseimmunity because of the synchronous detection. The output signal is fedinto a differentiator and threshold detector circuit as previouslydescribed. These circuits send a trigger signal to the controller whenthe detected signal amplitude drops at a rate sufficient for thedifferentiator to have an output exceeding the threshold level.

FIG. 24 illustrates a power supply and firing system suited for use inthese two alternative arrangements. The power supply generates plus andminus 15-volt levels, as well as a 5-volts level. The capacitor in thefiring circuit is charged by a secondary input winding on the powertransformer. This arrangement provides for isolation of the systemground from the machine ground and avoids the need to step up powersupply voltage to the capacitor voltage as accomplished by boostregulator 1750. However, the capacitor charge voltage becomes dependenton the line voltage, which is somewhat less predictable.

The charging circuit for the capacitor is regulated by an enable linefrom the controller. By deactivating the charging circuit, thecontroller can monitor the capacitor voltage through an output to an AIDline on the controller. When the capacitor is not being charged, itshould discharge at a relatively know rate through the various paths toground. By monitoring the discharge rate, the controller can insure thatthe capacitance of the capacitor is sufficient to burn the fusiblemember. The trigger control from the controller is used to fire the SCRto burn the fusible member.

With any of the above electronic subsystems, it is possible to avoidtriggering in the event metal or metal-foiled materials are cut bylooking for the amplitude of the signal, or the rate of change,depending on the system, to fall within a window or band rather thansimply exceeding or falling below a certain threshold. Moreparticularly, when metal is cut, the detected signal will drop to almostzero, and will drop within a single cycle. Thus, the controller orthreshold detection circuitry can be configured to look for amplitudechange of somewhat less than 100%, but more than 10% as a trigger event,to eliminate triggering on metal or other conductive work pieces whichwould normally substantially completely ground the signal.

It should be noted that, although not essential, all of the describedembodiments operate at a relatively high frequency—above 100 khz. Thishigh frequency is believed to be advantageous for two reasons. First,with a high frequency, it is possible to detect contact more quickly andsample many cycles of the waveform within a short period of time. Thisallows the detection system to look for multiple missed pulses ratherthan just one missed pulse, such as might occur due to noise, to triggerthe reaction system. In addition, the higher frequency is believed toprovide a better signal to noise ratio when cutting green wood, whichhas a lower impedance at lower frequencies.

As described above, the present invention provides a band saw which issubstantially safer than existing saws. The band saw includes a safetysystem adapted to detect the occurrence of a dangerous condition, suchas a person accidentally touching the moving blade, and to stop movementof the blade to prevent serious injury to a user. The band saw may beused to cut wood, plastic, or other non-conductive material.

The band saw also may be modified for use in the meat cutting industry.In that case, the detection system would be modified so that a user ofthe band saw would wear a glove with one or more interior wires on whichan electrical signal is induced. When the blade cuts into the glove andcontacts the interior wires, the blade would ground the wires and thedetection subsystem would detect that the signal on the wires hadchanged. The reaction system would then trigger as described above.

While several particular exemplary embodiments have been described andillustrated, it will be appreciated that many different modificationsand alterations may be made within the scope of the invention.

1. A band saw comprising: a motor; at least two spaced-apart wheels,where at least one wheel is driven by the motor; a band blade extendingaround the wheels, where the band blade is electrically isolated; signalgeneration circuitry adapted to generate an electrical signal; acapacitive coupling adapted to capacitively couple the signal generationcircuitry to the band blade to transfer at least a portion of theelectrical signal to the band blade; and detection circuitry thatmonitors the electrical signal on the band blade for changes indicativeof a dangerous condition between a person and the band blade, where thedetection circuitry triggers some action to mitigate the dangerouscondition when the changes indicative of the dangerous condition aredetected.
 2. The band saw of claim 1 further comprising a secondcapacitive coupling adapted to capacitively couple the detectioncircuitry to the band blade, and where the detection circuitry monitorsthe electrical signal on the band blade through the second capacitivecoupling.
 3. The band saw of claim 1 where at least one wheel is mountedfor rotation on a shaft, where the capacitive coupling includes twospaced-apart conductors, and where at least a portion of the shaft isone of the two conductors and a charge plate adjacent the shaft is theother of the two conductors.
 4. The band saw of claim 3 furthercomprising a second capacitive coupling adapted to capacitively couplethe detection circuitry to the band blade, where the detection circuitrymonitors the electrical signal on the band blade through the secondcapacitive coupling, where the second capacitive coupling includes twospaced-apart conductors, and where the shaft is one of the twoconductors and a charge plate adjacent the shaft is the other of the twoconductors.
 5. A band saw comprising: a motor; a band blade driven bythe motor, where the band blade is electrically isolated; signalgeneration circuitry adapted to generate an electrical signal; couplingmeans for capacitively coupling the signal generation circuitry to thecutting tool to transfer at least a portion of the electrical signal tothe cutting tool; and detection means for monitoring the electricalsignal on the cutting tool for changes indicative of a dangerouscondition between a person and the cutting tool, and for triggering someaction to mitigate the dangerous condition when the changes indicativeof the dangerous condition are detected.